Powder coating compositions which may be used for powder coating applications such as powder painting are advantageous in that they eliminate the disadvantages associated with the use of solvents in liquid paint systems, as for example, the liquid paint system disclosed in U.S. Pat. No. 2,857,354. Powder coating paint compositions are curable by heat in such a way that little, if any, volatile material is vaporized or evaporated into the environment. This, of course, is substantially different from liquid paint systems where the liquid paint vehicle is volatilized during the drying of the paint.
Processes for preparing powders for various applications are disclosed, for example, in U.S. Pat. Nos. 3,429,921, 3,730,930 3,737,401 and 3,770,848.
In the production of metal particle-containing powder coating compositions, there has been a problem in the past with degradation of the metallic ingredient. Since the metallic ingredient is used primarily for decorative purposes, as, for example, automobile top coats, where it is necessary that the finish have a metallic look, any degradation of the metal particle or flake presents a serious aesthetic problem.
It is known to produce metal particle-containing powder coating compositions by grinding metal particle-containing resin sheets to form pellets. However, this grinding operation tends to degrade the metal particles contained within the resin.
A prior art method for preparing metal particle-containing powder coating compositions is disclosed, for example, in U.S. Pat. No. 3,739,049. A particularly efficacious method is disclosed in U.S. Pat. No. 3,303,111 wherein such powders are prepared by spray-drying acrylic polymer solutions.
West German Pat. No. 2,419,330 discloses a process for producing powder coating compositions having a fine particle size. This process comprises spraying a solution of polymer in the form of a mist or fine droplets, contacting the mist or fine droplets with a precipitant for the polymer, precipitating the polymer in fine particle size upon such contact, and recovering the fine powder particles produced.
A difficulty involved with mamy of the processes for producing powder coatings is that such processes tend to produce powder particles many of which have become agglomerated because of the inability to keep the particles separate when produced in a liquid bath. Agglomeration is to be avoided, especially when thin film applications are desired.
The search has continued for improved processes for producing metal particle-containing powder coating compositions wherein the metal praticles remain substantially undegraded and wherein the powder coating compositions are in substantially non-agglomerated form.